In a post from 1999, Oscar Gaddy pointed out that all the various bluing processes did the same thing which was to grow a film of iron oxide on the metal surface.
The color variations run from the first temper colors produced by an oxide film 1/2 wavelength of light thick to blue-black to black as the film becomes thicker. The temper colors actually repeat as the oxide film goes from 1/2 to 1 to 1-1/2(and so on) wavelenghts of light. However, the thicker film also grows more opaque and thus absorbs more and more light reflected from the metal surface (as opposed to the oxide surface).
Re: Carbonia blue, a 1937 Winchester employee in the finishing shop said that their "machine bluing" process used sperm oil and bone charcoal in the rotating retort. He said that the smoke produced by the oil was what caused the coating to form. They had to be careful to not let too much air into the furnace or the oil smoke would catch fire. They got two heats per 12hr. day. Small parts bluing used pine tar instead of sperm oil. Some charcoal bluing was still being done at that time but he doesn't elaborate. Hot salts bluing(Du-Lite) began to be phased in in 1939.
The above is from "Winchester, The Way it Really Was" by P. Muerrle
OB
